High intensities of laser pulses are required for many applications, such as e.g. in biomedicine or material processing. With good beam quality (M2<1.2) and pico- or femtosecond pulses, this generally means pulse energies in the range of 1-10 μJ in order to exceed the typical ablation or processing threshold energy densities and to allow efficient process control.
The pulsed laser systems used for such applications, more particularly for industrial applications, which are often embodied as recoverable or regenerative amplifiers, increasingly require the ability to be able to provide laser pulses on demand. This is indeed a feature already integrated as standard for typical lasers of the nanosecond class, i.e. having characteristic pulse durations in the range of a few to a few tens of nanoseconds. With the maturation of applications which can advantageously or solely be operated specifically with ultrashort pulses, i.e. with pulse durations in the range of a few to a few tens of pico- and femtoseconds, in the future “pulse-on-demand” operation will become an indispensable feature for these lasers, too. This ability means the retrievability of pulses having predefined desired energies and/or desired pulse durations as necessary or depending on the presently needed requirements. Typical requirements, which exist nowadays especially in industrial application, are for example    a. The setting of repetition rate and pulse energy during processing. These are usually not very time-critical in current systems in the prior art and can be assumed to be adiabatic with respect to the system time constants of the laser.    b. The start of a pulse train with a specific repetition rate and pulse energy with TTL signal, wherein the switching edge has to lie between two pulses of the amplified pulse train.    c. The pulse energy boost of the first pulses is intended to be small, e.g. <10% of the target value of the pulse energy. Depending on the process, even higher energy boosts can be accepted, but in some processes only minimal or else no deviations whatsoever are permissible.    d. Finally, applications are increasingly becoming known in which settings of pulse repetition rate and pulse energy can or have to be formed continuously, e.g. from zero up to the maximum value of the repetition rate (0−fmax), and in a time-critical manner, i.e. with fast setting times, e.g. <1 μs.
The last two points c. and d. represent the critical requirements and cannot be achieved in ultrashort pulse laser systems in the prior art. Admittedly there are technical solutions with which very good time resolution and thus “on-demand” operation or retrievability of pulses can be achieved. This is typically realized with highly repetitive oscillators, a downstream pulse selector, followed by one or more linear post-amplifiers. However, the very high gain results in an energy boost (transient processes) during switch-on or state changes in the repetition rate of the pulse selector. These are far above the values regarded as permissible.
As minimum requirements to be realized for such laser systems with pulses retrievable according to demand and having a predefined duration, temporal separation (repetition rate) and energy, the following thresholds can be specified.    1. The pulse energy boost of the first pulses after the beginning of a pulse burst is intended to be only small, which typically means a deviation of <<10% from the target value of the pulse energy.    2. The pulse repetition rate and pulse energy are intended to be able to be varied continuously from zero up to the respective maximum value and moreover in a time-critical manner, i.e. with time scales <<1 μs.
The prior art discloses various approaches which involve measuring the output signal or the coupled-out laser pulse. A loss modulation of the laser pulse to be amplified can then be effected on the basis of this signal. However, these approaches thus presuppose the generation of a pulse and therefore take effect only for the subsequent pulses, such that immediate and instantaneous influencing of the retrievable pulse to be amplified does not take place.
Consequently, it is an object of the present invention to provide an improved laser system and a corresponding method for generating retrievable laser pulses having a predefined pulse duration and/or pulse energy.
It is a further object to provide such a laser system which is designed such that the system is controlled by open-loop or closed-loop control such that even in the first retrieved pulses only a small pulse energy boost occurs and continuously and instantaneously variable pulse repetition rates and pulse energies are made possible.